really depends on how busy the test centre you want to take it at is.
I know mine in a large city in Canada was backed up by about 2 months. but that's not to say that every once in a while random days would pop up as available because (I suppose) people cancelled or rescheduled their test.
I rescheduled mine about a week before for a time later in the day. I'm not sure exactly what you are asking, but No, I believe you can schedule a writing time for only 2 days in advance.
You need to get the confirmation letter from the ADA before you can register with prometric. The confirmation letter has a lifespan of a year I thinkk. So do that ASAP, then see how backed up your prometric location is. And decide from there.